Just curious as to why on some aircraft, the Allison 501-D//T56 was mounted on the top of the nacelle vs. the bottom on others. The two that come to mind are the Lockheed L-188/P-3 and the Convair 580, vs. a C-130. I've noticed on other turboprop aircraft (with engines such as the PWC PT-6 and PW120), the intake is on the bottom. Near the rear of the intake duct, is a bypass door that allows FOD (ice, snow, junk off the ramp, etc) to "fall out" and thus not get sucked into the engine where it can cause damage.

I'm just curious as to why the engine intake was on the top of these aircraft despite said advantages of a bottom intake.

Occasionally the inlet would become blocked on a L-188 by a sheet of plastic while the engine was running. This would require changing the engine because the turbulent airflow would result in first stage compressor blade fatigue failures. To let the engine continue in service will eventually result in messy failure, usually sooner than later.

Quoting Northwest727 (Thread starter):Near the rear of the intake duct, is a bypass door that allows FOD (ice, snow, junk off the ramp, etc) to "fall out" and thus not get sucked into the engine where it can cause damage.

As I recall, this feature requires something pretty unique to the P&W designs, namely, a flow reversal Technically, the PT6 is mounted "Backwards" compared with most other turboprop engines, so the intake air comes in the engine's "arse" so to speak (and my apologies to the Brits for stealing your particular dialect). This is also the reason for the usual dual exhaust ducts in a PT6 installation: the flow is split since you can't just pipe it straight out the back.

If you walk up to a turbine Aero Commander 690, for example, and look up its (smile-shaped) intake duct, you can see the compressor blades of the Garrett TPE-331 as I recall

Quoting KELPkid (Reply 6):As I recall, this feature requires something pretty unique to the P&W designs, namely, a flow reversal Technically, the PT6 is mounted "Backwards" compared with most other turboprop engines, so the intake air comes in the engine's "arse" so to speak (and my apologies to the Brits for stealing your particular dialect). This is also the reason for the usual dual exhaust ducts in a PT6 installation: the flow is split since you can't just pipe it straight out the back.

I know about the flow reversal with the PT-6, but the PW120 is a straight-through kind of turboprop. As I recall from doing Dash 8 ground school once, the doors should be left open, despite that they cause a marginal loss in power.

Quoting vzlet (Reply 7):I'm not positive you're still refering to T56s here, but if so, it's my understanding that that door (the one open on the #3 nacelle in the below photo) is for an oil cooler.

I was referring to the T56; I thought that opening on the bottom was this bypass door...I guess not.

Quoting Northwest727 (Thread starter):Near the rear of the intake duct, is a bypass door that allows FOD (ice, snow, junk off the ramp, etc) to "fall out" and thus not get sucked into the engine where it can cause damage.

That is an oil cooler exhaust door.

Quoting Northwest727 (Thread starter):I'm just curious as to why the engine intake was on the top of these aircraft despite said advantages of a bottom intake.

Different designs of the nacelle, not the engine itself. In your two photos you have two different nacelle designs due to one having the MLG enclosed within the nacelle and one that does not.

The first photo is of a plane with dual wheel MLG which must fit inside the nacelle. Move the intake (it is a simple S-curve) to the top of the nacelle provides more room on the underside to store that dual wheel with minimal drag increase. This is also a low-wing design so having the intake on the top of the nacelle helps keep the overall plane design lower to the ground. The second photo is of a plane with no storage requirements and a high wing mount location so the intake can be anywhere and since the nacelle is already "hung" below the wing, putting the intake there smooths out the upper wing surface.

The C-2 photo shows a plane with a single-wheel MLG on a high-wing mount design. The single-wheel does not take up much space (rotates 90 degrees) and the intake S-curve easily fits in front of the wheel well without changing the frontal area of the nacelle.

Bottom line is that it all depends upon what the plane designer is trying to accomplish. The large intake is for air to the engine and is a simple S-curve with the engine mostly in-line with the propeller [not quite, but pretty close]. That gives airplane designers plenty of flexibility in how they design the nacelles.

Quoting ZANL188 (Reply 3):probably not the best configuration for a short or medium range airliner with a low wing and built in airstair...

The C-2 was a modification to the E-2 design and was never designed for "transits." The "Hummer" was designed to place the radar in a position and loiter for long periods of time. The C-2 replaced the C-1 (much smaller piston engine plane) as the Carrier On-board Delivery (COD) plane. Being able to get on/off a ship being much more important than transit speeds.

Quoting Northwest727 (Reply 8):I was referring to the T56; I thought that opening on the bottom was this bypass door...I guess not.

Oil cooler exhaust door --note that it is always located near the smaller oil cooler intake. The T-56 exhausts bleed air out the oil cooler exhaust in order to draw cooling air into the oil cooler during ground operations. The bleed air cuts off during flight and the oil cooler exhaust door (normally) closes automatically.